MIC's generally comprise a substrate carrying signal lines and/or electronic devices on one side and a ground plane on the other. A second substrate and ground plane is added in the case of a stripline circuit. Whenever signals are coupled from one MIC to another within a package or from a coaxial cable to an MIC inside a package, there is a problem of providing an electrically adequate connection between the two ground planes or the ground plane and the outer conductor of the cable.
Direct physical connections generally require close tolerance machining or other measures which are incompatible with the goals of lowering the cost and increasing the modularity of systems including MIC's.